A known magnetic resonance imaging (MRI) scanner 10 is shown in FIG. 1. A patient 12 is slid axially into a bore 14 of a superconducting magnet 16, and the main magnetic field is set up along the axis of the bore, the Z-direction. Magnetic field gradients are set up, for example, in the Z-direction, to confine the excitation of magnetic resonant (MR) active nuclei (typically hydrogen protons in water and fat tissue) to a particular slice in the Z-direction, in the horizontal X and the vertical Y-directions (as shown in FIG. 1), to encode the resonant MR nuclei in the plane of the slice. A radio frequency (RF) transmit coil (not shown) applies an excitation pulse to excite the protons to resonance, and an RF receive coil arrangement comprising an array of receive coils 18, 20 picks up relaxation signals emitted by the disturbed protons.
To encode/decode received signals in the Y-direction, the signals are detected in the presence of a magnetic field gradient, termed a frequency encode or read-out (R.O.) gradient, to enable different positions of relaxing nuclei to correspond to different precession frequencies of those nuclei about the direction of the main magnetic field due to the influence of the gradient. The data is digitized, and so for each RF excitation pulse, a series of digital data points are collected, and these are mapped into a spatial frequency domain known as k-space (FIG. 2). Each RF pulse permits at least one column of digital data points to be collected. The set of data points acquired during one read-out event (for example, one column of FIG. 2) is commonly referred to as a “line” of data.
To encode/decode the received signals in the X-direction, after each RF pulse has been transmitted and before data is collected with the read-out gradient applied, a magnetic field gradient in the X-direction is turned on and off. This is done for a series of magnitudes of magnetic field gradients in the X-direction, one RF pulse typically corresponding to a different magnitude of gradient in the X-direction. On the k-space matrix shown in FIG. 2, the columns of data points correspond to data collected at different magnitudes of phase-encode (PE) gradients.
Generally, the field of view imaged by the magnetic resonance imaging scanner 10 depends on the spacing of the data points in the phase-encode and read-out directions, and the resolution of the image depends on how far the points extend in each direction, i.e., how large the maximum phase-encode gradient is, and on the magnitude of the read-out gradient combined with the duration of data collection. Conventionally, the data collected by the RF receive coils 18, 20 is subject to a two-dimensional fast Fourier transform to produce a pixelated spatial image. A slice image A is shown in FIG. 3, which corresponds to area A of the patient 10 shown in FIG. 1. FIG. 3 implies that the spacing of data points in the phase-encode gradient direction is sufficient to image the whole of the circle shown in FIG. 1.
Between each RF pulse, there is a certain minimum pulse repetition time, and the collection of a complete set of phase encode data implied by FIG. 2 may therefore take an undesirably long time. Parallel imaging is an MR imaging technique that uses multiple detectors (phased array coils) to partially replace phase encoding. Each coil measures a set of k-space data points that differ from each other because of the different spatial location and sensitivity of the coils, so the total number of unique measurements is a multiple of the number of coils. For example, if N data points are needed to make an image, these can be obtained from N measurements using one coil or from (N/R) measurements using R coils. Thus parallel imaging is quicker by a factor of up to R. This reduces acquisition times by decreasing the number of phase-encoded lines of k-space that must be acquired. Usually the phase encode data points sampled in parallel imaging are more widely spaced so they cover the same area but with reduced density. Practical implementation of parallel imaging includes SENSE (sensitive encoding) which operates in the image domain, and SMASH (simultaneous acquisition of spatial harmonics) that operates in the k-space matrix.
Navigation refers to an MR imaging technique that is applied during the scan to determine if patient motion has occurred, typically respiratory motion. Navigation uses multiple measurements of a reference part of the anatomy to detect positional changes. If such changes are detected, the affected data points resulting from the motion are corrected or re-acquired. The parallel imaging technique SMASH has been proposed as a way of doing navigation by using previously acquired data to predict the data that have yet to be acquired. Predictions can be adversely affected by systematic errors arising from SMASH data processing and also random errors from low signal to noise ratio (SNR), which reduce the accuracy of the navigation.
Partial Fourier is a technique that reduces the amount of data that is acquired to reduce the MR scan time. Partial Fourier does this by applying certain processing steps (phase correction) that allow one to make the assumption that one half of the k-space data is almost identical to the other half. Therefore, only a little over half the data is needed to make an image.